Access Type

Open Access Dissertation

Date of Award

January 2022

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Chemistry

First Advisor

Claudio N. Verani

Abstract

The molecular electronics field is a multidisciplinary area that addresses the utilization of molecules as the basic unit of electronic components. Depending on the characteristics of the molecules, they can function as transistors, wires, switches, and memory cells. This field has attracted significant interest from the scientific community due to the increasing number of issues associated with the miniaturization process of microelectronic devices and the desirability of using molecules in electronic circuits. The research work discussed in this dissertation thesis is focused on the design and development of terpyridine-based metallosurfactant as prospective candidates for molecular electronics. Since rectifiers are pivotal in electrical circuits, incorporating the functionality of rectifiers into molecules is an exciting research area. Asymmetric molecular species with an electron donor (D) and electron acceptor (A) groups are particularly attractive in this regard. Therefore, a series of ligands and iron(II), cobalt(II), ruthenium(II), and copper(II) metallosurfactants with [D-A] molecular architecture to analyze the current rectification phenomena. The synthesis and characterization of the redox, electronic, surface properties, and investigation of the characteristics of the thin films of the metallosurfactants are presented in great detail. The charge transfer behavior of the molecules was studied in electrode| Monolayer film| electrode type molecular junctions. In molecular rectifiers, the electron transfer occurs through the involvement of frontier molecular orbitals. Hence energetic match between the Fermi energy level of the electrode and the frontier molecular orbital energy level of the metal complex is crucial in the design of molecular rectifiers. In an attempt to obtain this energetic compatibility, a ruthenium terpyridine-based metallosurfactant associated with redox-active aminocatechol ligand was designed. A catechol-based ligand is pivotal to this research work because of its capability to display multiple electrochemical states such as quinone, semiquinone, and catechol. A combination of experimental and theoretical studies was performed to examine the electronic and redox properties of the complex. The NMR, UV-visible, cyclic voltammetry experiments present clear evidence of the presence of a ruthenium(III) metal center linked with a semiquinone moiety of the catechol-ligand. The redox and electronic properties indicated an excellent correlation with the theoretically calculated values. Additionally, the calculated molecular orbital energy diagram and the molecular orbital plots of the complex show about 45% of the HOMO orbital is localized on the metal center while about 70% of LUMO is located on the aminocatechol ligand. The Fermi-LUMO energy gap of the complex was shown to be 0.70 eV suggesting the plausibility of asymmetric electron transfer through the LUMO level of the molecule. Furthermore, to facilitate the formation of Langmuir-Blodgett films at the air-water interphase, long alkyl chain and tert-butyl groups are introduced into the ligand framework. The data suggested the formation of homogenous monolayers at the air-water interphase, which were further transferred onto solid substrates. To study the electron tunneling properties of the metallosurfactant, devices with electrode| LB monolayer| electrode were constructed, and an asymmetric current vs. voltage response with a maximum rectification ratio of 32 was observed. Therefore, this study demonstrated the importance of redox-active catechol ligands in modulating the LUMO-Fermi energy gap of potential molecular rectifiers. Several approaches can be used to modulate the frontier molecular orbital energies of a molecular species. Among these methods, the incorporation of push and pull type substituents to the ligand scaffold has shown to have a significant effect on LUMO-Fermi and HOMO-LUMO energy difference. Therefore, a study was conducted to analyze the impact of electron-withdrawing nitro groups on the redox, electronic, surface, and electron transfer properties of metal complexes. A series of ruthenium(II) complexes with 4-octadecyloxy phenyl terpyridine ligand as the donor moiety was designed and synthesized. The terpyridine, 4-phenyl nitro terpyridine, phenanthroline, and 5-nitro phenanthroline were used as the acceptor group of complexes 1-4, respectively. All complexes showed interesting electronic and redox properties. The electrochemical data suggest that the incorporation of a nitro group introduces an additional reduction peak to the cyclic voltammogram at a lower potential. According to the density functional theory (DFT) calculations, this process is affiliated with the reduction of the nitro group and corresponds to the LUMO level of the molecular species. Moreover, the fragment molecular orbital analysis also indicated that the LUMO level of nitro-containing complexes is localized in the nitro group. The molecular orbital diagram of the complexes demonstrates that the HOMO-LUMO energy difference can be reduced by incorporating electron-withdrawing substituents into the ligand. Additionally, the complexes with nitro groups have a lower LUMO-Fermi energy gap than their unsubstituted counterparts. The bis-terpyridine complexes showed poor surface behavior, attributed to the imbalance between hydrophilic and hydrophobic components of the molecule. In contrast, the phenanthroline-based complexes showed moderate amphiphilic behavior with the formation of a well-ordered monolayer at air-solid interphase. The spectroscopic characterization of the thin film confirms that the identity of the molecular species is not altered during the deposition process. As expected, the metallosurfactant with 5-nitro phenanthroline as the acceptor ligand showed unidirectional electron transfer through the molecule, while the complex with unsubstituted phenanthroline ligand indicated a molecular insulator like behavior with a considerably poor current response. Therefore, this research work exhibits the significance of electron-withdrawing substituents in the modulation of redox, surface properties, and the HOMO-LUMO energy gap of the molecular species. In addition to the ligand framework, the metal center also has a significant impact on modulating the frontier molecular orbital energy levels of metal complexes. Therefore, a series of symmetric bis terpyridine iron(II), cobalt(II), and ruthenium(II) complexes were synthesized, and their charge transfer properties were investigated. This research work was based on the hypothesis that asymmetric placement of electroactive moiety of the metallosurfactant between conductive electrodes would lead to asymmetric electron transfer through the molecule. The structural properties of metallosurfactants were analyzed using FT-IR, 1H-NMR, and mass spectroscopy methods, while the electronic and electrochemical properties were evaluated through UV-visible and cyclic voltammetry experiments. Isothermal compression data propose that all three metallosurfactants form stable monolayer films at the air-water interface with comparatively higher collapse pressures. Thin films of each complex were transferred onto different solid substrates and further characterized via infrared reflection absorption spectroscopy (IRRAS), mass spectroscopy, UV-visible spectroscopy, and atomic force microscopy imaging techniques. The molecular devices were prepared by sandwiching monolayer films of metallosurfactants between two gold electrodes to study the probability of electron transfer through molecules. The iron(II) complex shows an asymmetric current-voltage response with rectification, while the cobalt(II) exhibits a symmetric current response through the molecule. This behavior is related to the energetic compatibility between HOMO and LUMO levels with the Fermi energy level of the gold substrate. Conversely, the ruthenium(II) complex displays a significantly lower current, indicating a molecular insulator-like behavior. Due to the complexity associated with the development of robust Copper complexes, copper-based materials are not well-established in the molecular electronic field. However, copper complexes have several attractive qualities, such as the ability to exhibit multiple oxidation states, abundance, and affordability, which makes those attractive for molecular electronic applications. Therefore, a previously developed copper-aminopyridine complex, [CuII(aminopyridineOC18)2ClO4]ClO4, was used to analyze the effect of the molecular architecture of the deposited thin films on the redox, composition, and electron tunneling properties. This research work was done collaboratively with Dr. Izabella Brand at the Pure and Applied Chemistry Department at Carl von Ossietzky Universität in Oldenburg, Germany. The solution-state redox properties of the complex was analyzed in acetonitrile and dichloromethane solutions using glassy carbon and gold as the working electrodes. To study the electrochemical properties of monolayers of the copper complex, LB thin films were deposited as X and Z type monolayers on various substrates, where the hydrophobic and hydrophilic group is attached to the surface of the substrate, respectively. The electrochemical properties indicate that LB monolayers of both X and Z orientations show a reduction peak ca.-0.6 V; however, the X-type LB monolayer film exhibit a larger captive current than Z-type monolayers. The XPS analysis of X and Y-type LB films on gold electrodes also demonstrates significant changes in its composition, which concluded that the aminopyridineOC18 ligand had incorporated a -C=O group over time. This dissertation research work demonstrates the application of terpyridine-based iron(II), cobalt(II), and ruthenium(II) as molecular electronic components. The ligands and metal complexes were designed to merge electrochemical and amphiphilic properties to facilitate the construction of nanoscale molecular devices with redox-active molecular species. Moreover, this research project discussed different strategies to modulate frontier molecular orbital energies of the complexes that aid in obtaining desired electron tunneling properties through the molecule. Finally, this dissertation project enabled the development of molecular rectifiers that exhibit asymmetric electron transfer through the molecule.

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